Seoul, South Korea(CNN)South Korea's first big Halloween celebration since the end of Covid restrictions turned to tragedy on Saturday night, when at least 151 people, mostly teenagers and young adults, died as partygoers surged through a narrow alley in a popular nightlife district in Seoul, officials said.

Authorities are still investigating what caused the incident, but Choi Seong-bum, chief of the Yongsan-gu Fire Department, said it was a "presumed stampede" and that many people fell, injuring at least 82.

The dead included at least 19 foreign nationals, including people from Iran, Norway, China and Uzbekistan, he said.

South Korean President Yoon Suk Yeol convened an emergency meeting in the early hours of Sunday, and later visited the scene to receive briefings from emergency officials.

Addressing the nation, he called a national period of mourning "until the handling of the accident is concluded."

"A tragedy that should not have happened occurred in the middle of Seoul last night on Halloween," Yoon said. "I pray for those who died in an unexpected accident and hope that the injured will recover quickly."

Tens of thousands of revelers had poured into the Itaewon nightclub district on Saturday night to enjoy South Korea's first Halloween celebration since crowd limits and face mask rules imposed by the Covid pandemic were lifted.

Witnesses said that even before the chaos broke out, partygoers were packed so tightly in the narrow streets that it was difficult to move around.

"I saw people going to the left side and I saw the person getting to the opposite side. So, the person in the middle got jammed, so they had no way to communicate, they could not breathe," Witness Sung Sehyun told CNN. He said the space was like a "jammed subway."

Video posted to social media showed people performing compressions on other partygoers lying on the ground as they waited for medical help.

"We saw a scene from a movie... like things happening during a war," witness Park Jung-Hoon, 21, told Reuters. "They were doing CPR here and there and people were rushing in as nothing was being controlled. It was completely out of control."

Authorities received the first reports of people "buried" in crowds around 10:24 p.m. on Saturday night.

Yonhap News Agency reported that some people had suffered from "cardiac arrest," attributing the statement to fire authorities. Emergency officials assisted at least 81 people in Seoul's Itaewon neighborhood reporting "difficulty breathing." But early suggestions of a gas leak or fire were discounted.

Police closed off the area and social media videos showed people wearing Halloween costumes lying in the streets and on stretchers as first responders rendered aid and queues of ambulances formed to take away the injured.

Dozens of people were transferred to nearby facilities, said Choi Jae-won, the head of Yongsan Health Center. The bodies of the victims were transferred to multiple hospital mortuaries, authorities addeed.

The Seoul city government received reports of missing people as friends and families searched for people known to be at the event who didn't come home.

On Sunday, police officers scanned the sidewalk for personal belongings and pieces of identification as they tried to determine the final number of injured and dead.

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